1. Field of the Invention
This invention generally relates to a photo-coupler apparatus used as a relay, and more particularly, to a photo-coupler apparatus which has an output MOSFET at the secondary side and is used under the condition in which the current flowing through the conducting electrode of this output MOSFET is limited below a certain value.
2. Description of the Prior Art
In FIG. 1, the circuit structure of a prior art photo-coupler apparatus is shown.
This photo-coupler apparatus is comprised of the following: a light emitting diode 1; a photo-diode array 2; a discharging circuit (a resistor) 3; an NPN transistor 4 utilized to limit the gate voltage of an output MOSFET mentioned later below a certain value; a resistor 5 for detecting an output current; and an output MOSFET 6.
Receiving light from light emitting diode 1, photo-diode array 2 generates a voltage which is used to turn on output MOSFET 6. Thus, by turning on or off of the current supplied to light emitting diode 1, MOSFET 6 turns on or off, permitting this apparatus to work as a relay.
In the apparatus shown in FIG. 1, resistor 5 converts the output current I1 into a voltage which is applied to the base-emittor of NPN transistor 4. Then, a collector current flows through transistor 4 as a function of this base-emittor voltage. As the output current increases, the collector current increases. Once the collector current has approached the photo-current value supplied by photo-diode array 2, the operational point of photo-diode array 2 varies. This results in the decrease of the gate voltage of output MOSFET 6. Due to this feedback effect, output current I1 does not exceed a certain value. The protection against excessive current is thus realized in this apparatus.
The output current limited as explained above is expressed by the following formula. EQU I1=Vt/R1.multidot.ln(Isc/Is) (1)
In this formula, I1 means the magnitude of the output current, Vt means the magnitude of thermal voltage, R1 means the resistance value of resistor 5, Isc means the magnitude of output current from photo-diode array 2, and Is means the saturation current of NPN transistor 4. Thermal voltage Vt is obtained by the following equation: EQU Vt=k.multidot.T/q,
where k means the Boltzman constant, T means an absolute temperature, and q means the amount of electron charge.
There are three problems in the prior art photo-coupler apparatus described above.
First, the photo-coupler apparatus has a large heat loss. This is because resistor 5 is series-connected to the output line.
Base-emittor voltage Vbe of a conventional NPN transistor, which is used to limit the gate voltage of output MOSFET 6, is about 0.6 to 0.7 (V). Accordingly, a resistor having a value more than 4.OMEGA. is required, for example, in order to limit output current I1 below 150 mA (0.6 V /150 mA=4.OMEGA.). The on-resistance of output MOSFET 6 is about several to tens .OMEGA.. The value 4.OMEGA. cannot therefore, be ignored as compared with the on-resistance of output MOSFET 6. As a result, the heat loss of the photo-coupler apparatus becomes significant.
Second, as is clearly understood from equation (1), the output current limited by NPN transistor 4 has light quantity dependence against light emitting diode 1. The reason of this is as follows. When the output current from photo-diode array 2 increases, a large collector current flows through NPN transistor 4. This results in the increase of base-emittor voltage Vbe as well as the output current.
Third, the protection against heat runaway is not enough.
Thermal voltage Vt and resistor 5, which is made from Si containing diffused impurities, have positive temperature dependence, while saturation current Is of NPN transistor 4 has negative temperature dependence. Since saturation current Is is dominant among those, output current I1 has negative temperature dependence. About -0.6 mA/.degree.C. of temperature characteristic can be achieved by the prior art technique. This value is, however, not enough as compared with the value -1.5 mA/.degree.C. of the derating curve of a conventional photo-coupler apparatus.